3.1 Microstructure evolution during annealing

Microstructures of the original SG sample and LG sample are shown in Figures 1(a) and (b). Average grain sizes of the SG sample and LG sample are 5 and 50 μm, respectively. These two kinds of plates were cold-rolled at room temperature. Microstructures of these rolled plates were also shown in Figure 1. After 17% cold-rolling, plenty of shear bands are found in the SG sample with few twins, as shown in Figures 1(c) and (e). In these shear bands, grains are crushed and refined, showing complex morphologies compared with the grains besides the shear bands. It indicates that the plastic deformation concentrates in these shear bands. In contrast, in the LG sample, plenty of lamellar twins are found in grains. These twins are speculated as twins due to their special morphology which has been verified in many studies [28, 29].

Figure 1全屏查看

Microstructures of the original (a) SG sample and (b) LG sample and cold-rolled (c,e) SG sample and (d,f) LG sample

From the cold-rolled microstructure in Figure 1, it is seen that, during cold-rolling, the deformation mechanisms are different for SG plates and LG plates. It is because grain size has a great effect on the activation of twinning in Mg alloys [30]. In SG plates, twinning is suppressed due to their small grains where twinning is hard to be activated due to its high critical resolved-shear stress (CRSS). Dislocation slip is the dominating deformation mechanism during cold-rolling. Thus, few twins are found in SG-17%. Meanwhile, since the plastic deformation accommodated by dislocation slip is non-uniform, plenty of shear bands are produced in SG-17%. In LG plates, the CRSS of twinning is lower than that in SG plates. The stress concentration can also be produced easily in coarse grains to promote the activation of twinning. Therefore, twinning plays the dominating role during the cold-rolling of LG plates. Finally, high density of twins is produced in LG plates after 15% cold-rolling.

These cold-rolled plates with different microstructure characteristics are subjected to isothermal annealing at different temperature. Microstructures of the annealed SG-17% are shown in Figure 2 and the average grain size is measured and shown in Figure 3(a). The microstructure and grain size show gradual evolution with the increase of annealing temperature. When the annealing temperature is 150 ℃, the average grain size is 2.9 μm, smaller than that of the original plate. Some small grains are found along the shear bands. It indicates that recrystallized grains nucleate firstly in these shear bands during annealing, showing a shear band induced recrystallization mechanism. The high stored energy in these shear bands should be responsible for such phenomenon because the plastic deformation concentrates in these shear bands during cold-rolling. With the increase of annealing temperature, these recrystallized grains grow gradually to consume the matrix. Equiaxed grains are obtained when the annealing temperature is 300 ℃, and the average grain size is 6.2 μm. With the further increase in annealing temperature, the grain size increases gradually and it is 12.5 μm after 500 ℃ annealing.

Figure 2全屏查看

Microstructures of the annealed SG-17% at different temperature (Annealing temperature is marked at the top-right corner)

Figure 3全屏查看

Average grain size of annealed (a) SG-17% and (b) LG-15% at different annealing temperature

Figure 4 shows the microstructure of annealed LG-15% at different temperature and the average grain size is plotted in Figure 3(b). Static recrystallization also occurs during the isothermal annealing, but the annealed LG-15% shows different microstructure evolution. When annealing temperature is lower than 180 ℃, no obvious recrystallized grain is found in the microstructure. It indicates that the stored energy in LG-15% is lower than that in SG-17%. When the annealing temperature is 200 ℃, some small recrystallized grains are observed along twins. In Mg alloy, twinning can rotate the crystal orientation by 56.3°. After twinning, basal slip and twinning can be easily activated in twinning zones [8]. Thus, during cold-rolling, strong strain concentration is produced in twinning zones and promotes the nucleation of recrystallized grains. Then, during annealing, recrystallized grains nucleate firstly along twins, showing a twin induced recrystallization mechanism. With the increase of annealing temperature, these twins are devoured gradually with the growth of recrystallized grains. When annealing temperature is 350 ℃, equiaxed grains are obtained and the average grain size is 13.2 μm. Comparing the grain size in Figure 3, it is seen that the average grain size of annealed LG-15% is higher than that of annealed SG-17% at different annealing temperature.

Figure 4全屏查看

Microstructures of the annealed LG-15% at different temperature (Annealing temperature is marked at the top-right corner)

3.2 Texture evolution during annealing

EBSD was conducted on the as-rolled and annealed AZ31 Mg alloy plates to reveal the texture evolution during isothermal annealing. Figure 5 shows the IPF maps and (0001) pole figures of SG-17% and LG-15%. In IPF maps, grains are colored depending on their crystal orientation and the standard IPF map at the top-right corner of Figure 5(a). It should be noted that some black zones are not identified due to high strain concentration in these zones. From the IPF maps and (0001) pole figures, it is seen that, both SG-17% and LG-15% have a strong basal plane texture where c-axis of grains is parallel with ND of the plates.

Figure 5全屏查看

(a, b) Inverse pole figure (IPF) maps and (c, d) 0001 pole figures of (a, c) SG-17% and (b, d) LG-15%

Figure 6 shows the IPF maps and (0001) pole figures of annealed SG-17%. When annealing temperature is low, some zones are also not identified. When annealing temperature is 400 ℃, all grains are identified and equiaxed grains are obtained. It is consistent with the microstructure evolution in Figure 2. It is seen from the (0001) pole figures, after annealing at different temperature, these samples still own a strong basal plane texture. The texture has small difference compared with the as-rolled SG sample in Figure 5(c). It indicates that, in annealed SG-17%, the nucleation and growth of recrystallized grains have a small effect on the final texture.

Figure 6全屏查看

Inverse pole figure (IPF) maps and (0001) pole figures of annealed SG-17% at different temperature: (a) 200 ℃; (b) 270 ℃; (c) 400 ℃; (d) 500 ℃

IPF maps and (0001) pole figures of annealed LG-15% are shown in Figure 7. For the SG-17%, some zones are not identified when the annealing temperature is low. Equiaxed grains are obtained at high annealing temperature. However, the colors of grains, which indicate different crystal orientation, are richer than that of annealed SG-17%. It is also seen from (0001) pole figures where great difference is found. When annealing temperature is low, these annealed samples have strong basal plane texture for the LG-15%. It is because these fine recrystallized grains have a small effect on overall texture. When LG-15% is annealed at high temperature, a random basal texture is observed compared with the as-rolled LG-15%. The basal poles deviate from the top and bottom sides of the (0001) pole figures. It means that the nucleation and growth of recrystallized grains from twinning zones can weaken the basal texture of Mg alloy.

Figure 7全屏查看

Inverse pole figure (IPF) maps and 0001 pole figures of annealed LG-15% at different temperature: (a) 200 ℃; (b) 270 ℃; (c) 400 ℃; (d) 500 ℃

3.3 Mechanical properties of annealed plates

In order to reveal the effect of isothermal annealing on mechanical properties of cold-rolled SG and LG samples, firstly, microhardness measurement was conducted on the annealed SG-17% and LG-15% at different annealing temperature with prolonged annealing time. Microhardness distribution at different annealing temperature is shown in Figure 8. After cold-rolling, microhardness values of the SG and LG samples are 75HV and 64HV, respectively. During isothermal annealing, microhardness decreases gradually due to static recrystallization. With the prolonged annealing time, the microhardness variation can be divided into two stages. At the first stage, the microhardness decreases quickly with the prolonged annealing time and it reaches a steady value at the second stage. In addition, the steady value decreased gradually with the increasing annealing temperature. And the steady value of annealed SG-17% is higher than that of annealed LG-15% at each annealing temperature.

Figure 8全屏查看

Micro-hardness variation with the annealing time for (a) SG-17% and (b) LG-15%

During cold-rolling, the SG and LG plates experienced severe plastic deformation at room temperature. However, the SG plate has smaller grain size and higher rolling reduction, so working hardening is stronger in SG plates. Microhardness of the SG-17% is higher than that of the LG-15%. When the as-rolled plates are subjected to isothermal annealing, the stored energy in the rolled plates is in favor of the nucleation and growth of recrystallized grains. Once static recrystallization occurs in the plates, the working hardening can be consumed quickly. Therefore, microhardness of the as-rolled plates decreases quickly at the first stage of annealing. After that, recrystallized grains grow gradually with the prolonged time. However, the grain growth is restricted at each annealing temperature, so microhardness reaches a steady value at the second stage. In addition, the final grain size increased gradually with the increasing annealing temperature. Thus, the steady microhardness decreases gradually due to the weaker refinement strengthening effect.

Compression tests are conducted on the original, cold-rolled and annealed plates. Figure 9 shows the compressive true stress-strain curves of the original SG and LG plates. Since the SG sample has a smaller grain size than LG sample, yield strength and ultimate compressive strength of the SG sample are higher than those of LG sample both for ND and TD compression. When compressing along TD, the stress-strain curves show a concave shape. It attributes to the basal plane texture in the original SG and LG plates where twinning can be activated easily when compressing along TD. Such concave stress-strain curves are the typical curves where twinning dominates the compressive strain [31, 32].

Figure 9全屏查看

Compressive true stress-strain curves of original SG sample and LG sample with loading direction along ND and TD

Figure 10 shows the compressive stress-strain curves of the as-rolled and 500 ℃ annealed plates. After cold-rolling, yield strength and ultimate compressive strength are improved significantly due to working hardening. After 500 ℃ annealing, they decrease significantly due to static recrystallization. Meanwhile, the compression failure strain shows significant improvement both for ND and TD compression tests. The increment of failure strain for ND compression is much higher than that for TD compression. For annealed SG-17%, the ND failure strain increases from 9.7% to 31% while the TD failure strain increases from 16.6% to 20%. And for annealed LG-15%, the ND failure strain increases from 19.5% to about 50% while the TD failure increases from 20.7% to 32.5%. It is also noted from Figure 10 that, in ND compression, the final failure strain for annealed LG-15% is much higher than that of annealed SG-17%; however, their true stress is almost at the same level. It means that they have experienced different strain hardening behavior during compression tests. Different texture evolution during annealing should be responsible for their great difference in mechanical properties, and it will be discussed in the following section.

Figure 10全屏查看

Compressive true stress-strain curves of as-rolled and 500 ℃ annealed (a) SG-17% and (b) LG-15% with the loading direction along ND and TD

4.1 Static recrystallization behavior

During cold-rolling, plenty of shear bands and twins are produced in SG plates and LG plates, respectively. These zones have severe strain concentration with high stored energy. Thus, when these rolled plates are subjected to annealing, static recrystallization will occur firstly in these zones. As shown in Figure 11, grains are colored depending on their conditions and the fraction of deformed grains, substructure and recrystallized grains are marked at the top-right corner. When annealing temperature is 200 ℃, the fraction of recrystallized grains is small, corresponding to the nucleation stage. With the increase of annealing temperature, recrystallization is promoted due to higher imported energy. Thus, the volume fraction of recrystallized grains increases gradually with the decrement of deformed grains.

Figure 11全屏查看

Recrystallization distribution maps of the annealed (a, c, e, g) SG-17% and (b, d, f, h) LG-15% at different temperature: (a, b) 200 ℃, (c, d) 270 ℃, (e, f) 400 ℃ and (g, h) 500 ℃ (Fraction of recrystallized grains, substructure and deformed grains are marked at the top-right corner)

In addition, it is noted that the recrystallization process shows different characteristics for annealed SG-17% and LG-15%. At low annealing temperature (200 ℃ and 270 ℃), the fraction of recrystallized grains for SG-17% is 16% and 45%, respectively, while it is 2% and 36% for LG-15%. The annealed LG-15% shows lower recrystallization fraction and higher deformation fraction. In contrast, at high annealing temperature (400 ℃ and 500 ℃), the fraction of recrystallized grains of LG-15% is 65% and 86%, respectively, higher than that of SG-17%. The fraction of deformed grains for LG-15% is only 2% after 500 ℃ annealing. These results indicate that SG-17% and LG-15% experience different recrystallization process where shear bands and twins have different effect on static recrystallization behavior. At the earlier stage of static recrystallization, shear bands can promote the nucleation and growth of recrystallized grains, showing a higher effect on static recrystallization. It is also seen in Figures 2 and 4 that, at low temperature, recrystallized grains are firstly observed along shear bands while it is few in twin zones. At the later stage, in annealed LG-15%, recrystallization grains from twins can grow quickly due to the weaker obstruction from grain boundaries. Thus, twins have a greater effect than shear bands at the later stage of static recrystallization.

In order to further reveal their different recrystallization behavior for SG-17% and LG-15%, the recrystallization fraction is estimated depending on the microhardness variation in Figure 8, using the following equation [33-35]:

(1)

where is the microhardness of the as-rolled plates, is the microhardness of the annealed plates at time and is the final microhardness. It should be declared that complete static recrystallization is achieved hypothetically for each annealing temperature. On this way, the recrystallization degree can be evaluated through the value of f, as shown in Figure 12. Clearly, recrystallization fraction increased gradually with the prolonged annealing time. SG-17% has a higher increase rate than that of LG-15% at each temperature. It has been reported that the annealing temperature , annealing time and recrystallization activation energy satisfy the following equation: [36, 37]

(2)

Figure 12全屏查看

Variation of recrystallization fraction with the prolonged annealing time for the annealed (a) SG-17% and (b) LG-15% at different annealing temperature

where is a constant and is gas constant. Using the common relationship , then:

(3)

In order to receive the activation energy for SG-17% and LG-15%, the variation of with is plotted in Figure 13 where is the annealing time when the recrystallization fraction is 50%. Clearly, the 1/T and show linear relationship where the slope of this line is 2.3R/Q [36]. Linear fitting is conducted on 1/T distribution lines, and the slopes for the SG-17% and LG-15% are 1.91×10^-4 and 2.26×10^-4, respectively. Then, the activation energy can be calculated as 100 kJ/mol and 85 kJ/mol for the SG-17% and LG-15%, respectively. Thus, during isothermal annealing, the activation energy of SG-17% is higher than that of LG-15% even through the SG-17% had a slightly higher rolling reduction. It means that the twins in Mg alloy are more helpful for the nucleation and growth of recrystallized grains during isothermal annealing.

Figure 13全屏查看

Variation of with the where is the annealing temperature and is the time when the recrystallization fraction is 50%

4.2 Effect of static recrystallization on texture variation

As stated in Section 3.2, during isothermal annealing, basal texture of SG-17% and LG-15% has different evolution with the increase of annealing temperature. It indicates that the static recrystallization from shear bands and twins has different effect on the basal texture of Mg alloy. The reason is revealed through the texture variation of the recrystallized grains, as shown in Figure 14. The recrystallized grains are extracted from the IPF maps in Figures 6 and 7, where (0001) pole figures of these recrystallized grains are plotted. Clearly, when annealing temperature is 200 ℃, the recrystallized grains are just forming with small volume fraction. Moreover, recrystallized grains of annealed SG-17% have strong basal texture while a random orientation distribution is found for annealed LG-15%. Thus, once these recrystallized grains nucleate and grow during static recrystallization, they can produce grains with different crystal orientation in annealed SG-17% and LG-15%.

Figure 14全屏查看

(a-d, i-l) IPF maps and (e-h, m-p) (0001) pole figures of the recrystallized grains for the annealed (a-h) SG-17% and (i-p) LG-15% at different annealing temperature: (a, e, i, m) 200 ℃, (b, f, j, n) 270 ℃, (c, g, k, o) 400 ℃ and (d, h, l, p) 500 ℃

It has been previously reported that twins can rotate the crystal orientation of Mg alloy by 56.3° [38, 39]. Even though the activation of basal slip and twinning in twinning zones can further change their orientation, the orientation in twinning zones still deviates from the c-axis//ND orientation [28]. Thus, these recrystallized grains from twinning zones have a random orientation. With the increase of annealing temperature, these recrystallized grains with random orientation grow gradually. A weak basal texture can be obtained in the annealed LG-15%. However, for SG-17%, recrystallized grains still have a c-axis//ND orientation in the shear bands. At different annealing temperature, the annealed SG-17% still has a strong basal texture as the as-rolled plate.

4.3 Effect of static recrystallization on mechanical properties

During isothermal annealing, microstructure and texture are modified due to static recrystallization which has a great effect on the mechanical properties of SG-17% and LG-15%. As shown in Figure 15, after cold-rolling, the yield strength is improved significantly with the sacrifice in compressive failure strain due to strain hardening effect. After 500 ℃ annealing, the yield strength shows significant decrement with a great improvement in failure strain due to recrystallization softening effect. However, the variation of yield strength and ductility shows great difference for SG-17% and LG-15% due to their different microstructure and texture characteristics.

Figure 15全屏查看

(a) Yield strength and (b) compressive failure strain for the as-rolling and annealed SG-17% and LG-15%

Firstly, their yield strength shows different reduction after 500 ℃ annealing, as shown in Figure 15(a). For annealed SG-17%, the ND compressive yield strength decreases from 303 MPa to 186 MPa, with a 38.6% reduction while the TD compressive yield strength has a 65% reduction. In contrast, for annealed LG-15%, both ND and TD compressive yield strength have a high reduction, about 56%. As stated above, the annealed SG-17% has a strong basal texture. Pyramidal slip and twinning should be, respectively, the dominating deformation mechanisms during ND and TD compression. Since pyramidal slip has a high CRSS, the ND yield strength keeps a high value and shows a low reduction, while a high reduction is found for TD yield strength due to the low CRSS of twinning [40]. However, for annealed LG-15%, a weak basal texture is found after 500 ℃ annealing. c-axis of grains deviates from the ideal c-axis//ND orientation. Such orientation is in favor of the activation of basal slip, so basal slip can be activated easily during ND and TD compression [16, 36]. The CRSS of basal slip is low in Mg alloy, so both the ND and TD compressive yield strength show significant reduction after 500 ℃ annealing.

Secondly, after 500 ℃ annealing, SG-17% has a smaller grain size than LG-15%, so it has a higher yield strength depending on refinement strengthening effect. However, the ND-yield strength of SG-17% is much higher than that of LG-15% which is beyond the effect of grain size [41, 10]. Furthermore, as shown in Figure 15(b), both ND and TD compressive strains of annealed LG-15% are much higher than that of annealed SG-17% in spite of its small grain size. It is then obvious that, in addition to grain size, the weak basal texture in annealed LG-15% should also be responsible for its low yield strength and high compressive ductility. With a weak basal texture, basal slip can be activated easily in annealed LG-15%. Thus, it has a low ND-yield strength due to the low CRSS of basal slip. Meanwhile, in Mg alloys, basal slip can accommodate more plastic deformation than prismatic slip and twinning, so high compressive strain is found when compressing along the ND and TD of annealed LG-15%.

Thirdly, concerning the stress-strain curves in Figure 10, although the annealed SG-17% has a much higher ND-yield strength than LG-15%, they achieve a similar stress level during ND compression tests. It ascribes to their different strain hardening effect where different deformation mechanisms are activated. Pyramidal slip is the dominating deformation mechanism when compressing along the ND of SG-17%. The activation and movement of pyramidal slip is difficult in Mg alloys, so weak strain hardening is observed. In contrast, basal slip can be activated easily to accommodate the ND compressive strain for annealed LG-15% due to its weak basal texture. Strong strain hardening can be produced due to the universal basal slip. Meanwhile, LG-15% has a longer strain hardening stage than SG-17% due to its higher compressive strain. As a result, the maximum stress has a similar level for annealed SG-17% and LG-15% in ND compression.